The present invention relates to a method for producing a semiconductor crystal made of silicon (Si) atoms, germanium (Ge) atoms and carbon (C) atoms.
A mixed crystal semiconductor (SiGe) made of Si and Ge is well known as a material which forms a heterostructure with Si and from which an ultra-fast semiconductor device can be fabricated. However, SiGe has a lattice constant greater than Si. Thus, when an SiGe layer is epitaxially grown on a Si layer, a very large compressive strain is caused in the SiGe layer. For this reason, when the thickness of the SiGe layer deposited on the Si layer exceeds a thickness (critical thickness), a phenomenon, in which the strain in the SiGe crystal is relieved with generation of defects such as dislocations in the crystal, occurs. Even though no defects are observed immediately after the crystal growth, when a thermal treatment, which is essential for semiconductor processing, is carried out, defects are apt to be caused particularly in the SiGe crystal having a high Ge content. In other words, the SiGe crystal has low resistance to heat, which is an unfavorable property from the viewpoint of fabricating devices. In addition, an energy band offset appears only at the valence band of the SiGe layer around an Si/SiGe heterojunction. Therefore, since carriers are confined only in the valence band, in forming an MOS transistor in which the SiGe layer in an Si/SiGe structure is used as a channel, only a p-channel transistor having positive holes as carriers can be fabricated.
With regard to compensating for the above drawbacks of an Si1xe2x88x92xGex crystal, it is a mixed crystal semiconductor (SiGeC) made of Si, Ge and C that has been considered particularly important in recent years. C is an element having a smaller atomic radius as compared to Si and Ge. By introducing C into a crystal, the crystal can have its lattice constant decreased and its strain lessened. Since this allows the amount of strain accumulated in the crystal to be reduced, its resistance to heat can also be increased. In addition, when Ge and C contents are increased (several tens percent of Ge and several percent of C) in an SiGeC layer around an Si/SiGeC heterojunction, the offset of energy band can be produced at both the valence band and conduction band of the SiGeC layer. In this case, carriers are confined in both the conductive band and the valence band, thus enabling the fabrication of not only a p-channel transistor but also an n-channel transistor.
Further, C introduced into an SiGe layer functions effectively to suppress the diffusion of an impurity such as boron. In this case, an SiGeC crystal having a C content of about 0.1% or less is used.
The SiGeC crystal cannot be formed by a method performed in a thermally equilibrium state, such as a melt growth method. Therefore, as will be described later, a crystal growing technique performed in a thermally non-equilibrium state, such as a molecular beam epitaxy (MBE) process or a chemical vapor deposition (CVD) process, for example, has been conventionally utilized for forming the SiGeC crystal.
The MBE process is a process in which source atoms are evaporated and transported to a substrate at 300-500xc2x0 C. under ultrahigh vacuum conditions so as to grow a crystal on the substrate. However, this process has drawbacks; sources needs to be changed, the crystal cannot be formed on a face with minute unevenness, and it is difficult for the substrate to have a large diameter, for example. Hence, this process is not suitable for the mass-production of the SiGeC crystal.
Next, as the CVD process, a rapid thermal chemical vapor deposition (RT-CVD) process or a limited reaction processing (LRP) is usually used. The CVD process is a process in which a crystal is grown on a heated substrate by introducing source gases with a large quantity of hydrogen in a medium to high vacuum. In forming the SiGeC crystal, silane (SiH4) is mainly used as a Si source, GeH4 is used as a Ge source, and monomethylsilane (SiH3CH3), ethylene (C2H4) or acetylene (C2H2), for example, is used as a C source. Conventionally, the crystal is grown under temperature conditions of 550-600xc2x0 C. as in the case of growing an SiGe layer.
Problems to be Solved
The SiGeC crystal, in which strain and band offset can be controlled much more freely, is a material which can realize more various devices of higher quality than an SiGe crystal. However, it is not easy to produce the SiGeC crystal because of the following properties thereof.
First, the solid solubility of C atoms in Si and Ge is very low (about 1017/cm3 and about 108/cm3 in Si and Ge, respectively, in a thermally equilibrium state). Thus, it is impossible to produce an SiGe crystal having a high C content (percent order) by a melt growth method, for example, performed in a thermally equilibrium state.
Also, due to their properties, it is likely for C atoms to enter not only the lattice sites but also the interstitials of the crystal. The C atoms that have entered the crystal interstitials become a carrier recombination center, which presumably adversely affects the characteristics of devices.
Further, since the C atoms tend to selectively bond with Si atoms in the SiGeC crystal, crystalline silicon carbide (SiC) is apt to be produced locally. Moreover, amorphous-SiC-like structures can be formed. Moreover, depending on crystal growth conditions, precipitates of C atoms are likely caused. Such local structures result in decrease in crystallinity.
Hence, it is very difficult to epitaxially grow, on a Si layer, an SiGeC crystal having a relatively high C content and homogeneity (with no local structure such as SiC crystal, for example) applicable to semiconductor devices, i.e., an SiGeC layer of high quality with a relatively high C content and a low defect density.
For example, it was difficult to form an SiGeC crystal of high quality with a high C content and a low defect density even by the CVD process performed in a thermally non-equilibrium condition.
An object of the present invention is providing a method for growing, on a substrate, an SiGeC crystal applicable to a semiconductor device, which has homogeneity (with no local structure such as SiC crystal) and good crystallinity.
A method for producing a semiconductor crystal in accordance with the present invention includes the steps of: (a) introducing a source gas containing silicon (Si), a source gas containing germanium (Ge) and a source gas containing carbon (C) into a container in which a substrate is held; and (b) thermally dissolving the source gases, thereby producing a semiconductor crystal containing Si atoms, Ge atoms and C atoms on the substrate. In the method, the thermal dissolution step is carried out at a temperature of 490xc2x0 C. or less.
By the above method, it is possible to form, on the substrate, a semiconductor crystal containing Si, Ge and C, with good crystallinity.
In the method described above, the semiconductor crystal is formed by a thermal CVD process. By this method, a semiconductor crystal with good crystallinity can be efficiently formed on the substrate. Further, a semiconductor crystal with good crystallinity can also be formed on a substrate including a patterned member.
In the method described above, Si2H6 or Si3H8 is used as a source gas of Si to be contained in the semiconductor crystal. By this method, a growth rate of about 4-8 nm/min can be achieved for the semiconductor crystal even at a low temperature of 490xc2x0 C. or less. Hence, it is possible to mass-produce a semiconductor device including the semiconductor crystal having good crystallinity.